This is a multi-center collaborative study to determine the sensitivity, specificity, positive and negative predictive value of contrast enhanced spiral-computed tomography (spiral CT) scan for the diagnosis of acute pulmonary embolism (PE). PE is common, yet frequently undiagnosed and fatal. Ventilation/perfusion lung scans, the usual initial test are non- diagnostic in 72 percent of patients with suspected PE, and 57 percent of patients with subsequently proven PE. Pulmonary angiography is the definitive diagnostic test, but there is associated with morbidity, discomfort, cost, and lack of availability in community hospitals. Noninvasive leg tests, particularly venous ultrasonography, permit a strategy of management by identifying deep venous venous thrombosis (DVT) and potentially obviating the need for pulmonary angiography. Spiral CT is nearly a noninvasive test that offers the possibility of a definitive diagnosis of PE by showing the outline of the thrombus in a pulmonary artery. However, its utility in the diagnosis of PE is unknown. Even though spiral CT has not been adequately validated, its use is becoming widespread as a definitive diagnostic test for PE, or diagnostic arbiter for non-diagnostic lung scans in many hospitals. This cold lead to over treatment and under treatment, both of which have serious potential complications. This investigation will accurately evaluate the role of spiral CT scan in the diagnosis of PE by comparison with pulmonary angiography, V/Q lung scan in patients without prior PE, pulmonary angiography, or compression ultrasound of the lower extremities in patients with no prior deep venous thrombosis. PE will be diagnosed based on a high probability lung scan in patients with no prior PE, pulmonary angiography, or compression ultrasound of the lower extremities in patients with no prior DVT. DVT detected by venous ultrasound will serve as a surrogate for the diagnosis of PE. Pulmonary embolism will be excluded by pulmonary angiography or nearly normal lung scans with no adverse outcome (i.e. PE or DVT) on follow-up treated patients. Among patients with a low probability lung scan, PE will be excluded by 2 negative venous compression ultrasound studies at 1 week intervals followed by 6 months of follow-up with no treatment and no recurrent PE or DVT. The strengths of this proposal are: 1) the importance of the problem of PE in terms of lives lost from under-diagnosis, and major bleeding from over-diagnosis 2) potential applicability of a new technology for the diagnosis of PE that will have widespread availability, 3) protocol parallels standard diagnostic strategy without subjecting the patients to risk entirely for the acquisition of data.